Movie producers are continually becoming more concerned about movie piracy. In the past year, for example, more than 50 major movies were illegally copied and publicly released even before they came out in theaters, according to the Motion Picture Association of America (M.P.A.A.). The film industry lost a reported US$3.5 billion last year, and estimated losses may rise to US$5.4 billion this year. Illegally copied movies filmed during projection, with video cameras or camcorders and similar devices, are a significant contributing factor to revenue loss. While it may not be possible to completely eliminate theft by copying, it can be advantageous to modify image display and projection techniques so that pirated copies are of such low quality that they are of little or no commercial value.
It is known to provide a distinct symbol or watermark on an original still image as a means of image or copy identification, so as to enable authentication of a copy. As examples, U.S. Pat. No. 5,875,249 (Mintzer et al.), U.S. Pat. No. 6,031,914 (Tewfik et al.), U.S. Pat. No. 5,912,972 (Barton), and U.S. Pat. No. 5,949,885 (Leighton) disclose methods of applying a perceptually invisible watermark to image data as verification of authorship or ownership or as evidence that an image has not been altered. However, while such methods identify and validate image data, they provide no direct means of protection against copying an image, such as using a conventional scanner and color printer.
In contrast, U.S. Pat. No. 5,530,759 (Braudaway et al.) discloses providing a visible, color correct watermark that is generated by altering brightness characteristics but not chromaticity of specific pixels in an image. This approach could be objectionable if used for a motion picture, since the continuing display of a watermark on film could annoy an audience and adversely affect the viewing experience.
The above examples for still-frame images illustrate a key problem: an invisible watermark identifies but does not adversely affect the quality of an illegal copy, while a visible watermark can be distracting and annoying. With video and motion picture images, there can be yet other problems with conventional image watermarking. For example, U.S. Pat. No. 5,960,081 (Vynne et al.) discloses applying a hidden watermark to MPEG data using motion vector data. But this method identifies and authenticates the original compressed data stream and would not provide identification for a motion picture that was copied using a camcorder.
Other patents, such as U.S. Pat. No. 5,809,139 (Girod et al.), U.S. Pat. No. 6,069,914 (Cox), and U.S. Pat. No. 6,037,984 (Isnardi et al.) disclose adding an imperceptible watermark directly to the discrete cosine transform (DCT) coefficients of a MPEG-compressed video signal. If such watermarked images are subsequently recompressed using a lossy compression method (such as by a camcorder, for example) or are modified by some other image processing operation, the watermark may no longer be detectable.
These particular invisible watermarking schemes add a watermark directly to the compressed bit stream of an image or image sequence. Alternatively, there are other watermarking schemes that add the watermark to the image data itself, rather than to the compressed data representation. An example of such a scheme is given in U.S. Pat. No. 6,044,156 (Honsinger et al.), which discloses a spread spectrum technique using a random phase carrier.
However, regardless of the specific method that is used to embed a watermark, there is always a concern that a watermarking method might not be robust, that is, able to withstand various “attacks” that can remove or alter the watermark. Some attacks may be deliberately aimed at the underlying structure of a given watermarking scheme and require detailed knowledge of watermarking techniques applied, although most attack methods are less sophisticated, performing common modifications to the image such as using lossy compression, introducing lowpass filtering, or cropping the image, for example. Such modifications can be made when a video camera is used to capture a displayed motion picture. These methods present a constant threat that a watermark may be removed during the recording process.
The watermarking schemes noted above are directed to copy identification, ownership, or authentication. However, even if a watermarking approach is robust, provides copy control management, and succeeds in identifying the source of a motion picture, an invisible watermark may not be a sufficient deterrent for illegal copying.
As an alternative to watermarking, some copy deterrent schemes used in arts other than video or movie display operate by modifying a signal or inserting a different signal to degrade the quality of any illegal copies. The modified or inserted signal does not affect playback of a legally obtained manufactured copy, but adversely impacts the quality of an illegally produced copy. As one example, U.S. Pat. No. 5,883,959 (Kori) discloses deliberate modification of a burst signal to foil copying of a video. Similarly, U.S. Pat. No. 6,041,158 (Sato) and U.S. Pat. No. 5,663,927 (Ryan) disclose modification of expected video signals in order to degrade the quality of an illegal copy. As yet another example of this principle, U.S. Pat. No. 4,644,422 (Bedini) discloses adding a degrading signal to discourage copying of audio recordings. An audio signal having a frequency at and above the high threshold frequency range for human hearing is selectively inserted into a recording. The inserted signal is not detectable to the listener. However, any unauthorized attempt to copy the recording onto tape obtains a degraded copy, since the inserted audio signal interacts adversely with the bias oscillator frequency of a tape recording head.
The above-mentioned copy protection schemes deliberately inject a signal in order to degrade the quality of a copy. While such methods may be effective for copy protection of data from a tape or optical storage medium, these methods do not discourage copying of a motion picture image using a video camera.
As a variation of the general method where a signal is inserted that does not impact viewability but degrades copy quality, U.S. Pat. No. 6,018,374 (Wrobleski) discloses the use of a second projector in video and motion picture presentation. This second projector is used to project an infrared (IR) message onto the display screen, where the infrared message can contain, for example, a date/time stamp, theater identifying text, or other information. The infrared message is not visible to the human eye. However, because a video camera has broader spectral sensitivity that includes the IR range, the message will be clearly visible in any video camera copy made from the display screen. The same technique can be used to distort a recorded image with an “overlaid” infrared image. While the method disclosed in U.S. Pat. No. 6,018,374 can be effective for frustrating casual camcorder recording, the method has some drawbacks. A more sophisticated video camera operator could minimize the effect of a projected infrared watermark using a filter designed to block infrared light. Video cameras are normally provided with some amount of IR filtering to compensate for silicon sensitivity to IR.
Motion picture display and video recording standards have well-known frame-to-frame refresh rates. In standard motion picture projection, for example, each film frame is typically displayed for a time duration of 1/24th of a second. Respective refresh rates for interlaced NTSC and PAL video recording standards are 1/60th of a second and 1/50th of a second. Video camera capabilities such as variable shutter speeds allow close synchronization of a video camera with film projection, making it easier for illegal copies to be filmed within a theater. Attempts to degrade the quality of such a copy include that disclosed in U.S. Pat. No. 5,680,454 (Mead) and U.S. Pat. No. 6,529,600 (Epstein), which disclose use of a pseudo-random variation in frame rate, causing successive motion picture frames to be displayed at slightly different rates than nominal. Using this method, for example, frame display periods would randomly change between 1/23rd and 1/25th of a second for a nominal 1/24th second display period. Timing shifts within this range would be imperceptible to the human viewer, but significantly degrade the quality of any copy filmed using a video camera. The randomization proposed therein would prevent resynchronization of a video camera to a changed display frequency. While these methods may degrade the image quality of a copy made by video camera, they also have limitations. As noted in the disclosure of U.S. Pat. No. 5,680,454, the range of frame rate variability is constrained, since the overall frame rate must track reasonably closely with accompanying audio. Furthermore, a video camera can easily be modified to so that its frame rate tracks with the motion picture.
U.S. Pat. No. 5,959,717 (Chaum) also discloses a method and apparatus for copy prevention of a displayed motion picture work. The apparatus of U.S. Pat. No. 5,959,717 includes a film projector along with a separate video projector. The video projector can be used, for example, to display an identifying or cautionary message or an obscuring pattern that is imperceptible to human viewers but can be recorded using a video camera. Alternately, the video camera may even display part of the motion picture content itself. By controlling the timing of the video projector relative to film projector timing, a message or pattern can be made that will be recorded when using a video camera, but will be imperceptible to a viewing audience. This method, however, requires distribution of a motion picture in multiple parts, which greatly complicates film replication and distribution. Separate projectors are also required for the film-based and video-based image components, adding cost and complexity to the system and to its operation. Image quality, particularly for large-screen environments, may not be optimal for video projection, and alignment of both projectors to each other and to the display surface must be precisely maintained.
U.S. Published patent application No. 2002/0168069 describes an apparatus and method for displaying a copy-deterrent pattern within a digital motion picture in order to discourage recording of the motion picture using a video camera or other sampling recording device. The copy-deterrent pattern comprises a plurality of pixels within each frame of the digital motion picture, and the displayed pixel intensities are modulated at a temporal frequency using modulation characteristics deliberately selected to be imperceptible to human observers while simultaneously producing objectionable aliasing in any copy made using a video camera. There are drawbacks in this approach. During recording using a video camera the camera shutter can be set to open long enough to average out the modulation. For example, in the example shown in FIG. 12 of the reference, the frame rate is 24 frames per second, and the modulation is at 96 Hz. If the camera shutter speed is set at 1/48 sec, then there will be no or minimal alias effect. Furthermore, if sampling rate is fast enough (e.g. 96 Hz) then all the information will be recorded in the pirate copy and the modulation in pixel intensities can be removed by digital signal processing.
U.S. published application No. 2004/0033060 describes a method of modulation of a video signal with an impairment signal to increase the video signal masked threshold. With this arrangement, an impairment signal is applied to a version of the movie having a higher frame rate than normal. The impairment signal is designed to produce an apparent motion across the frame when applied at a high frame rate and less rapid apparent motion when viewed at a lower frame rate. When a person views the high frame rate movie, the brain is capable of summing rapid frames such that the impairment is not visible. When a lower frame rate video camera records such a signal, it will not capture all of the images, but rather only particular ones and as such the summing of successive images to produce the proper result will not be achieved. However, with recent improvements in CCD technology, the exposure period possible with video recorders is much longer. Where previously a 60 frame per second camera would have an exposure time that was much less than 1/60th of a second, new technologies allow exposures approaching the full 1/60th of a second duration. In such a case, the video camera would be able to perform an integration to sum successive rapid frames such that the slower frames are substantially correct.
Conventional methods such as those described above could be adapted to provide some measure of copy deterrence and watermarking for digital motion pictures. However, none of the methods noted above is wholly satisfactory, for the reasons stated.